Velocity, attenuation, confidence interval and Seki parameter of ODP Hole 118-735B gabbros (Appendix)

Compressional velocity and attenuation were measured as a function of depth in layer 3 gabbros from the recovered core at Ocean Drilling Program Site 735. High-frequency (400 kHz) acoustic transmission experiments were conducted on 117 minicore samples under room conditions of temperature and pressure on board the JOIDES Resolution. Although experimental errors were large in some deformed samples, the average of our measurements of 1000/Qp on these samples is 49 ± 32. Ultrasonic tests on a 1% porosity sample at elevated effective pressures from 10 up to 500 MPa showed an increase in Vp from 6.8 to 7.0 km/s and a decrease in 1000/Qp from 26 to 23. The small pressure dependence of attenuation in these low-porosity gabbros enables one to use measurements at room conditions for relative interpretation as a function of depth.
These measurements of 1000/Qp vs. depth show (1) an overall decrease in attenuation from lithologic Units II through V as a function of depth, (2) a 10-fold decrease in the average variance in the measurement in lithologic Unit II, and (3) an inverse relationship between average velocity and average attenuation. An explanation for these results is that tectonized rock fabrics have a significant effect on the measurement of attenuation and velocity. The future use of this simple shipboard operation to measure trends in attenuation as a function of depth may be effective to determine the degree of alteration and tectonism in similar, nonpressure-sensitive rocks.